It is known that a light amount (brightness) of a subject image, which is obtained by an image sensing optical system used in a camera which uses a silver halide film or an electronic camera, gradually decreases in proportion to the distance from an optical axis. This is called the limb darkening, which is inevitably caused by the cosine fourth power law, vignetting, or the like. An image sensed by an optical system with large limb darkening characteristic often becomes dark in its four corners and gives an unnatural impression.
In a conventional camera using a silver halide film, there is no other means but stopping down a lens, and/or adjusting exposure to light at the time of printing in accordance with exposure of a peripheral part of an image recorded on the film, for instance, for making limb darkening less conspicuous.
Moreover, in an actual optical system, it is inevitable that an optical axis of the optical system slightly deviates from a center of an image sensing element due to manufacturing errors. Since the center of the aforementioned limb darkening also becomes off the center of the image sensing element as the optical axis deviates from the center, darkening becomes intense in one corner on the image sensing element while darkening is eased in the other corner. In general, it is said that decrease in brightness toward the limb portion in a sensed image is less conspicuous when a decreasing rate is constant than when it is very large in one part while small in other part even if the difference between the light amounts at the central portion and the limb portion is the same. Since, in general, brightness drops rapidly outside an angle of view, off-centered limb darkening is extremely conspicuous and is likely to critically degrade quality of a photographed image.
In an image sensing apparatus using a photoelectric conversion image sensing element such as a CCD, which is represented by a digital camera, since a sensed image is recorded as numerical value data, a method for obtaining a satisfactory image by correcting the data is desired.